Heat-resistant cross-linked polyethylene insulated low-voltage electric wires for automobiles (hereinafter referred to as “AEXs”) that satisfy 120-degree heat resistance of the JASO Standards (Japan Automobile Standards Organization) are heat-resistant insulated electric wires used in wiring in vehicles such as automobiles. Insulated electric wires and electric cables (hereinafter, electric cables may also be referred to as “insulated electric wires”) used in wiring in vehicles are required to have good flexibility for ease of cable routing and space saving, and AEXs are also desired to have improved flexibility. As an insulated electric wire having good flexibility, for example, PTL 1 discloses an insulated electric wire including an insulating coating formed of a resin composition that contains, for example, a base resin containing a polypropylene resin, a propylene-α-olefin copolymer, and a low-density polyethylene resin, a metal hydrate, and a phenolic antioxidant, and a wire harness including the insulated electric wire.
Insulated electric wires for hybrid vehicles and electric vehicles that have been developed in recent years are required to include conductors having larger diameters so that a large current can be supplied. In order to manage an increase in the diameters of conductors, further improvements in flexibility are desirable. Furthermore, in order to manage generation of a large quantity of heat due to supply of a large current, improvements in heat resistance are desirable. PTL 2 discloses an insulating resin composition which enables production of an insulated electric wire that has flexibility and heat resistance good enough to meet the recent requirements described above and which can provide a sufficient water-cut-off performance (terminal water cut-off structure). The insulating resin composition contains a resin containing a first copolymer which is a copolymer of ethylene and an unsaturated hydrocarbon having 4 or more carbon atoms and which has a density of less than 0.88 g/cm3, and a second copolymer which is a copolymer of ethylene and an acrylic acid ester or a copolymer of ethylene and a methacrylic acid ester at a ratio represented by the first copolymer: the second copolymer (mass ratio) of 100:0 to 40:60; and 30 to 100 parts by mass of a flame retardant and 1 to 5 parts by mass of a crosslinking assistant relative to 100 parts by mass of the resin. PTL 2 further discloses an insulated electric wire that includes an insulating layer formed of a crosslinked polymer of this insulating resin composition and that has good flexibility, heat resistance, and water-cut-off performance (terminal water cut-off structure).